1. Field of the Invention
The present invention relates to the measurement of environmental parameters in an adverse environment and more specifically to the measurement of ultraviolet light ("UV") dosage in a chamber for UV curing of polymers.
2. Description of the Prior Art
UV curing is commonly used in the fabrication and finishing of polymeric products or components or products or components coated with polymeric materials. In the UV curing process, an uncured polymeric product or polymeric-coated product is carried through a curing chamber, past a UV curing lamp. For example, UV-cured products may include discrete parts, a web-like product, UV-cured inks and coatings, and filament-type products. The products may be carried on an open mesh belt or conveyor. Optical fiber is a filament-type polymeric product which may be cured in a UV curing process. In each of the examples, the UV radiation cures a polymeric compound to cross-link the composition in order to harden the compound, stabilize it, bind it to a substrate, or the like.
The ultraviolet radiation used in an ultraviolet curing process is provided by a high-intensity arc lamp enclosed in a quartz tube. The lamp is typically cylindrical and between four inches and four feet (about 10.2 cm to about 122 cm ) in length. These lamps have power ratings of about 150 to 600 watts per inch (about 59.06 watts per cm to about 236.22 watts per cm). Reflectors are commonly used to concentrate radiation emitted by the lamps onto the product. Several types of reflectors are used in the industry. For example, an ultraviolet lamp is placed at the focus of a parabolic reflector to produce uniform ultraviolet light intensity over a given area for curing discrete parts or web material. This arrangement is also useful in sterilizing a bath of liquid such as water. An elliptically-shaped reflector may be used for focusing ultraviolet light on a single, high-intensity strip for curing web material.
In curing a filament-type material, two opposed elliptically-shaped reflectors arranged to form the shape of an ellipse may be used to cure the filament on all sides simultaneously. The lamp is located at one focus of the ellipse and the fiber travels through a transparent quartz inerting tube in the curing chamber at the other focus of the ellipse. Due to the shape of the ellipse formed by the opposed reflectors, the reflectors focus radiation from the lamp at one focus of the ellipse onto the fiber at the other focus, 360.degree. around the fiber. In this way, the surface of the fiber facing away from the lamp receives radiation. The fiber is thereby cured on all sides simultaneously.
In all ultraviolet curing processes, it is critical to maintain a desired level or dose of radiation within a critical range in order to properly cure the material. An improper dose of radiation results in a defective product. For example, in curing ultraviolet curable ink, an excessive dose of radiation may cause cracking, discoloration or peeling, whereas an insufficient dose of radiation leaves the ultraviolet curable ink tacky, unstable, and improperly cured.
The dose of radiation is determined by the intensity of the ultraviolet light and the time of exposure of the material to the ultraviolet light. Several variables influence the intensity of ultraviolet radiation available within the chamber to cure the product. Two such variables are the performance of the ultraviolet lamp bulb and the reflectivity of the reflectors, each of which degrades over time. The useful operating life of the UV lamp is about 2,000 hours. Contamination in the curing process degrades components in the chamber and results from unlinked monomers and other vapors which are outgassed from the components and other materials in the curing chamber. Organic substances are deposited on the components in the chamber, including the ultraviolet lamp, the reflecting surfaces of the reflectors and the walls of the transparent inerting tube. Over time, the efficiency of the curing process is reduced. Although the exposure time for the product may be increased, by slowing the rate of the product passing through the chamber, the efficiency of the process is dramatically reduced and the cost of producing the ultraviolet cured product dramatically increased as a result of the contamination of the components. In addition, the degradation of the components may not be detected until a significant amount of product has emerged from the chamber in a defective state. Thus, detection of changes in these process variables is critical to assure the quality of an ultraviolet-cured product.
In order to maintain reasonably constant process parameters which within an ultraviolet curing chamber, as well as to detect problems in the process as they arise, the radiation intensity within the curing chamber is monitored. One method of monitoring ultraviolet intensity is to place a sensor on the conveyor which carries the product through the ultraviolet curing chamber. The sensor is thereby subjected to the same radiation as the product to be cured. A system utilizing this type of sensor is disclosed in U.S. Pat. No. 5,424,547 to Stark, et al. These sensors may also be positioned at the end of a glass rod or "light guide" that transmits ultraviolet light from the ultraviolet lamp to the sensor. However, it may be necessary to insert the light guide into the ultraviolet curing chamber during each measurement. Sensors used in these systems either contain sufficient circuitry to process and display the radiation data or the sensor is provided with sufficient memory to store the data for later processing and display by another device.
A sensor for measuring UV radiation within an inerting tube used in an optical fiber curing process is marketed by 4D Controls Ltd. under the name SOLATELL.RTM. Sola-Scope. That instrument comprises a hand-held stainless steel probe that may be inserted into an inerting tube. An exit iris at the end of the tube gathers UV data. The probe is manually rotatable in the inerting tube. No provision is made to index the probe to repeatable angular positions in the tube. The fiber curing process must be interrupted in order to take measurements.
The above systems require an operator to physically position the sensor and to take measurements and readings on a regularly scheduled bases. A typical manufacturing operation may include 20 or more UV curing chambers. In typical manufacturing operations involving numerous such ultraviolet curing chambers, the monitoring, recording and interpretation of measurement data at regular intervals may present significant difficulties.
A sensor permanently mounted to measure radiation in a UV chamber offers several distinct advantages. First, such a sensor need not be repositioned between readings; the repeatability of the sensor is thereby enhanced. Further, the taking of readings may be automated, assuring adequate monitoring of the process, and reducing the possibility that the process may be run with radiation levels outside the process limits.
A sensor may be permanently mounted in an ultraviolet chamber as disclosed in U.S. Pat. No. 4,665,627 to Wilde, et al. That patent describes a sensor mounted at one end of a tube extending into an ultraviolet chamber. The tube is aimed directly at the ultraviolet lamp and the sensor is mounted on the end of the tube outside the chamber. This system may automatically measure the output of the ultraviolet lamp at regular intervals and adjust the lamp intensity accordingly. Another type of sensor arrangement is disclosed by U.S. Pat. No. 5,418,369 to Moore, et al. for a UV curing system for curing optical fiber. This system uses an elliptical reflector and a quartz tube, as discussed above. The sensor senses the radiation through a series of holes in the reflector assembly.
In these systems, the sensor is subjected to the environment in the chamber by way of the sensor tube or holes in the reflector and therefore are exposed to contamination from the curing chamber. The sensor, like the other components in the system, degrades in accuracy as outgassed organic matter is deposited on the sensor or on a protective lens on the sensor. In addition, the ultra-violet radiation within a UV curing chamber can be as high as 200 times that of the sun. It would be desirable to protect the sensor from any unnecessary exposure to radiation. Thus, although some advancement has been made in monitoring and controlling process parameters in ultraviolet curing processes, there is room for significant improvement.